This invention pertains to an improved process for producing improved dispersion strengthened alloy metal products.
Dispersion strengthened metal products, such as copper dispersion strengthened with aluminum oxide, have many commercial and industrial uses. Welding electrodes, for example, require good electrical and thermal conductivities together with good strength and hardness at elevated temperatures. Dispersion strengthening has been recognized in the past as a method for increasing strength and hardness of metals. A solid solution alloy comprising a relatively noble matrix metal having relatively low heat or free energy of oxide formation and a solute metal having relatively high negative heat or free energy of oxide formation can be dispersion strengthened by heating the alloy under oxidizing conditions to preferentially oxidize the solute metal. This technique of oxidizing the solute metal to a solute metal oxide is known in the art as in situ internal oxidation or more simply internal oxidation.
Several processes for internal oxidation have been suggested. Commonly assigned U.S. Pat. No. 3,779,714, incorporated herein by reference, provides an improved alloy-oxidant mixture wherein the oxidant includes an oxide which releases oxygen to oxidize the solute metal of the alloy. A further improvement is set forth in commonly assigned U.S. Pat. No. 3,893,844 which provides improved dispersion strengthened metals by first recrystallizing the alloy powder prior to internal oxidation to increase the grain size of the alloy to a grain size at least as large as Grain Size No. 6 as measured by ASTM Test. No. E-112.
In commonly assigned U.S. Pat. No. 3,779,714, a dilute solid solution alloy powder is obtained by atomizing an alloy melt of matrix metal with a minor amount of solute metal wherein the melt is atomized by nitrogen and collected as powder in water. During the atomization process, however, oxygen in the atomization chamber tends to oxidize the solute metal on the surface of the alloy particles. For instance, a copper-aluminum alloy tends to become oxidized during atomization to form an aluminum oxide film on the particle surfaces. Further oxidation of the surface aluminum can occur upon the powder contacting water in the collection tank. The hot powder falling into the water generates steam in the atomization chamber which further contributes to the surface oxidation of the alloy particles. The accumulative effect of surface oxidation is the formation of a relatively thick aluminum oxide film on the surface of the alloy particles.
It now has been found that the surface oxide film formed on the alloy particles remains intact and can be detrimental in subsequent internal oxidation by forming a barrier to internal oxidation. Subsequent processing can inhibit interparticle bonding during subsequent fabrication of the powder into compacted fully dense parts obtained, for example, by hot forging and rolling. The final product can be weak and brittle. In a subsequent hot extrusion step, the powder particles tend to stretch out into elongated fibers thus improving interparticle bonding, although surface oxide film has been found to remain on fiber interfaces and causes diminished mechanical properties due to improper and incomplete interparticle or interfiber bonding. Elimination of the surface oxide has been found to substantially improve the mechanical properties of the dispersion strengthened alloy such as stress rupture strength as well as substantially improve the internal oxidation step for dispersion strengthening alloys. The surface oxide forms a barrier to the diffusion of oxygen into the alloy particles and, therefore, elimination of the surface oxide provides efficient, uniform, and effective oxidation of higher solute metal alloys. Significant improvements in the properties of internally oxidized alloys can be achieved by atomizing alloys in helium and collecting the alloy powder dry. These and other advantages will become more apparent by referring to the Detailed Description of the Invention.